Method of operating a dual motor drive and control system for an electric vehicle

ABSTRACT

A method for optimizing the torque applied by each motor of a dual motor drive system of an all-electric vehicle is provided, the torque adjustments taking into account wheel slip as well as other vehicular operating conditions.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/782,413, filed 18 May 18, 2009, which is a continuation-in-part ofU.S. patent application Ser. No. 12/322,218, filed 29 Jan. 2009, and acontinuation-in-part of U.S. patent application Ser. No. 12/380,427, nowU.S. Pat. No. 7,739,005, the disclosures of which are incorporatedherein by reference for any and all purposes.

FIELD OF THE INVENTION

The present invention relates generally to electric vehicles and, moreparticularly, to a dual electric motor drive system and correspondingcontrol system.

BACKGROUND OF THE INVENTION

The trend towards designing and building fuel efficient, low emissionvehicles has increased dramatically over the last decade, this trenddriven by concerns over the environment as well as increasing fuelcosts. At the forefront of this trend has been the development of hybridvehicles, vehicles that combine a relatively efficient combustion enginewith an electric drive motor.

Currently, most common hybrids utilize a parallel drive system, althoughthe implementation of the parallel drive system can vary markedlybetween different car manufacturers. In one form, illustrated in FIG. 1,power to wheels 101 is via planetary gears 103 and transaxle 105, thepower coming from either, or both, combustion engine 107 and electricmotor 109. A power splitter 111 splits the power from combustion engine107 between generator 113 and the drive system, i.e., gears 103, axle105 and wheels 101, the power split designed to maximize efficiencybased on vehicle needs. The electric power generated by generator 113,after passing through an inverter 115, is used to either provideelectricity to drive motor 109 or battery 117.

In hybrid system 100, motor 109 is the primary source of propulsion whenthe engine is relatively inefficient, for example during initialacceleration, when stationary, under deceleration or at low cruisingspeeds. Combustion engine 107 assists motor 109 in supplying propulsionpower when demands on the vehicle are higher than what can be met bymotor 109, for example during medium-to-hard acceleration,medium-to-high cruising speeds or when additional torque is required(e.g., hill climbing).

FIG. 2 illustrates the basic elements of another type of parallel drivesystem, often referred to as an integrated motor assist, or IMA, system.IMA system 200 utilizes a single electric motor 201 that is positionedbetween the combustion engine 203 and the drive system's transmission205, transmission 205 coupling power through axle 207 to wheels 209. Inthis system motor 201 serves dual roles; first, as a drive motor andsecond, as a generator. In its capacity as a generator, motor 201 iscoupled to battery pack 211 via inverter 213.

In hybrid system 200, engine 203 is the primary source of propulsionwhile motor 201 provides assistance during acceleration and cruising.During deceleration, motor 201 recaptures lost energy using aregenerative braking scheme, storing that energy in battery pack 211. Asa result of this approach, a smaller and more fuel-efficient engine canbe used without a significant lose in performance since motor 201 isable provide power assistance when needed.

Although in general hybrids provide improved fuel efficiency and loweremissions over those achievable by a non-hybrid vehicle, such carstypically have very complex and expensive drive systems due to the useof two different drive technologies. Additionally, as hybrids still relyon an internal combustion engine for a portion of their power, theinherent limitations of the engine prevent such vehicles from achievingthe levels of pollution emission control and fuel efficiency desired bymany. Accordingly several car manufacturers, including Tesla Motors, arestudying and/or utilizing an all-electric drive system.

FIG. 3 illustrates the basic components associated with oneconfiguration of an all-electric vehicle. As shown, EV 300 couples anelectric motor 301 to axle 303 and wheels 305 viatransmission/differential 307. A power control module 309 couples motor301 to battery pack 311.

FIGS. 4 and 5 graphically illustrate some of the performance differencesbetween a vehicle using a combustion engine as the sole propulsionsource, one using hybrid technology, and one using only a singleelectric motor. In the torque curves shown in FIG. 4, curve 401illustrates the narrow region over which a typical combustion engineprovides torque, and thus the reason why multiple gears are required toutilize such an engine efficiently. Curve 501 in FIG. 5 is thecorresponding power curve for the combustion engine. In a hybridconfiguration, the output from a combustion engine is combined with anelectric motor, thus combining the low speed torque provided by theelectric assist motor (curve 403) with that of the combustion engine(curve 401) to provide a dramatic improvement in low speed torque.Curves 405 and 503 illustrate the torque and power, respectively, ofsuch a combination. Curves 407 and 505 illustrate the benefits of a highoutput power, all electric drive system, specifically showing both thelow speed torque/power that such a system provides as well as the widespeed range over which such torque/power is available.

Although significant advancements have been made in the area of fuelefficient, low emission vehicles, further improvements are needed. Forexample, hybrid vehicles still rely on combustion engines for a portionof their power, thus not providing the desired levels of fuelindependence and emission control. Current all-electric vehicles,although avoiding the pitfalls associated with combustion engines, maynot have the range, power or level of traction control desired by many.Accordingly, what is needed is an improved all-electric vehicle drivesystem. The present invention provides such a system.

SUMMARY OF THE INVENTION

The present invention provides a method for optimizing the torqueapplied by each motor of a dual motor drive system of an all-electricvehicle.

In at least one embodiment of the invention, a method of operating anelectric vehicle traction is disclosed, the method comprising the stepsof computing vehicle speed, computing a total torque requirementrequest, splitting the total torque requirement request into optimalfirst and second motor torque requests, monitoring a wheel speed sensorand computing a wheel slip error, minimizing the wheel slip error andtransforming the optimal first and second motor torque requests intofirst and second motor torque commands, and controlling the first andsecond electric motors of the electric vehicle based on the first andsecond motor torque commands. The disclosed method may further compriseone or more monitoring steps, including; monitoring first and secondmotor speed sensors, monitoring a steering sensor, monitoring a brakesensor, monitoring an accelerator sensor, monitoring first and secondpower control module temperature sensors, monitoring energy storagesystem (ESS) temperature sensors, monitoring ESS voltage sensors, andmonitoring ESS current sensors. The disclosed method may furthercomprise the steps of computing first and second motor maximum availabletorque and limiting the optimal first and second motor torque requestsby the first and second motor maximum available torque and/or limitingthe first and second motor torque commands by the first and second motormaximum available torque. The disclosed method may further comprise thestep of computing optimal first and second motor flux commands.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a parallel drive system according to the prior art;

FIG. 2 illustrates a parallel drive system based on an IMA configurationaccording to the prior art;

FIG. 3 illustrates an all-electric drive system according to the priorart;

FIG. 4 graphically illustrates the torque curves for a combustionengine, a hybrid configuration and an all-electric drive systemaccording to the prior art;

FIG. 5 graphically illustrates the power curves for a combustion engine,a hybrid configuration and an all-electric drive system according to theprior art;

FIG. 6 illustrates the basic elements of a dual electric motor drivesystem in accordance with the invention;

FIG. 7 graphically illustrates the torque curves for a preferred dualmotor configuration;

FIG. 8 graphically illustrates the power curves for a preferred dualmotor configuration;

FIG. 9 illustrates the basic elements of a dual electric motor drivesystem in accordance with a first embodiment of the invention;

FIG. 10 illustrates the basic elements of a dual electric motor drivesystem in accordance with a second embodiment of the invention;

FIG. 11 illustrates the basic elements of a dual electric motor drivesystem in accordance with a third embodiment of the invention;

FIG. 12 illustrates the basic elements of a dual electric motor drivesystem in accordance with a fourth embodiment of the invention;

FIG. 13 illustrates the basic elements of a torque control system foruse with a dual electric motor drive system such as that shown in FIG.11;

FIG. 14 illustrates the basic elements of a torque control system for adual electric motor drive system similar to that shown in FIG. 13, withthe exception that each motor/power control module is coupled to aseparate ESS;

FIG. 15 illustrates the basic elements of the torque controller shown inFIG. 13;

FIG. 16 illustrates the algorithm used to calculate the optimal torquesplit between the two motors, without taking into account wheel sliperrors;

FIG. 17 illustrates the algorithm used to generate the look-up tableutilized by the optimal torque split unit;

FIG. 18 illustrates a block diagram of the traction control unit shownin FIG. 15; and

FIG. 19 illustrates the basic elements of the torque controller shown inFIG. 14.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In the following text, the terms “electric vehicle” and “EV” may be usedinterchangeably and refer to an all-electric vehicle. Similarly, theterms “hybrid”, “hybrid electric vehicle” and “HEV” may be usedinterchangeably and refer to a vehicle that uses dual propulsionsystems, one of which is an electric motor and the other of which is acombustion engine. Similarly, the terms “battery”, “cell”, and “batterycell” may be used interchangeably and refer to any of a variety ofdifferent rechargeable cell chemistries and configurations including,but not limited to, lithium ion (e.g., lithium iron phosphate, lithiumcobalt oxide, other lithium metal oxides, etc.), lithium ion polymer,nickel metal hydride, nickel cadmium, nickel hydrogen, nickel zinc,silver zinc, or other battery type/configuration. The term “batterypack” as used herein refers to multiple individual batteries containedwithin a single piece or multi-piece housing, the individual batterieselectrically interconnected to achieve the desired voltage and currentcapacity for a particular application. The terms “energy storage system”and “ESS” may be used interchangeably and refer to an electrical energystorage system that has the capability to be charged and discharged suchas a battery, battery pack, capacitor or supercapacitor. Lastly,identical element symbols used on multiple figures refer to the samecomponent, or components of equal functionality. Additionally, theaccompanying figures are only meant to illustrate, not limit, the scopeof the invention.

FIG. 6 illustrates the basic elements of a dual electric motor drivesystem 600 in accordance with the invention. As shown, power fromelectric motors 601 and 603 is sent to axle 605 viatransmission/differential assembly 607. In this configuration, althoughthere are two motors 601/603 coupled to axle 605, there is a single axlespeed, i.e., system 600 is not configured to allow independent drivespeeds at wheels 609 and 611. Motors 601 and 603 may operate at the samespeed, or different speeds, depending upon how each motor is coupled toaxle 605 via transmission/differential assembly 607. Note that as with aconventional vehicle, power may be coupled to one or both wheels viaaxle 605. For purposes of this simplified illustration, a singleESS/power control module 613 is shown coupled to both motors 601/603,however, as described in detail below, the inventor envisions poweringand controlling these two motors in a variety of ways and module 613 isonly meant to represent, not limit, such means. In the preferredembodiment of the invention, and as described in more detail below,preferably the operating characteristics of motors 601 and 603 aredifferent, thus allowing the overall drive train performance to beoptimized.

In a preferred embodiment of the invention, both motors 601 and 603 areAC induction motors. While the operating characteristics of the twomotors are selected on the basis of the desired drive train performance,in the exemplary and preferred embodiment illustrated in FIGS. 7 and 8,one of the motors (e.g., motor 601) is designed to have a relativelyflat torque curve over a wide range of speeds such that it may be usedto augment the output of the second motor (e.g., motor 603) at highspeeds, specifically in the range in which the torque of second motor603 is dropping off. FIGS. 7 and 8 illustrate torque and power curves,respectively, of motors 601 and 603 in such a configuration. Inparticular, curves 701 and 801 represent the torque and power curves,respectively, of motor 601 in this configuration while curves 703 and803 represent the torque and power curves, respectively, of motor 603.Curves 705 and 805 represent the torque and power curves, respectively,of the combination of motors 601 and 603.

It will be appreciated that there are numerous ways of coupling motors601 and 603 to axle 605 and as such, motors 601 and 603 may or may notrotate at the same rate for a given axle speed. For example, in oneembodiment, one of the motors is coupled via the sun gear oftransmission/differential assembly 607 while the other motor is coupledvia the ring gear.

FIGS. 7 and 8 illustrate that in at least one preferred embodiment, themaximum amount of torque from one of the motors (e.g., motor 601) issubstantially constant throughout the range of motor speeds, and as aresult the maximum amount of assist power increases as a function ofmotor speed (FIG. 8). This preferred embodiment applies to both themotoring and regenerating modes of operation. One benefit of thisapproach is that it can be used to compensate for torque fall-off athigher speeds, a characteristic typical of electric motors with limitedoperating voltage. Another benefit of significantly increasing the highspeed capabilities of a vehicle in accordance with the preferredembodiment of the invention is improved vehicle performance,specifically in the areas of top speed, high speed acceleration, andhill climbing abilities. Lastly, utilizing the dual motor approach ofthe present invention, in some configurations it is possible to achievea lower total motor weight than a single motor sized to provide similarcapabilities.

As previously noted, the curves shown in FIGS. 7 and 8 assume the use ofAC inductions motors even though this is not a requirement of theinvention. Curve 703 illustrates a characteristic common of many suchmotors, i.e., exhibiting a relatively flat peak torque at low speedswhich then drops off at higher speeds. As used herein, a motor's “basespeed” is defined as the speed at which the torque drops to 95% of theflat peak torque and will continue to drop after the base speed up tothe top speed under constant power source limits. Therefore, for curve703, this knee point occurs at a point 707 on the curve, leading to abase speed of approximately 7200 rpm. As used herein, a motor's “drivesystem base speed” is equivalent to the motor's base speed aftergearing, i.e., the motor base speed divided by the transmission gearratio. As described above and illustrated in FIGS. 7 and 8, preferablyone of the motors (e.g., motor 601) is designed to provide a much higherdrive system base speed than the drive system base speed of the othermotor (e.g., motor 603). For example, in one embodiment motor 601 isdesigned to provide at least a 50% higher drive system base speed thanthe drive system base speed of second motor 603.

The basic configuration illustrated in FIG. 6 provides a number ofadvantages over a single drive EV. First, the dual motor configurationcan be designed to provide superior performance, both in terms of anoptimized power curve and overall system efficiency, throughout a largerrange of speeds and a larger range of loads (i.e., load torques) whichare typical for a high performance vehicle. Second, by splitting theload between two motors, it is easier to keep the motors within thedesired operating temperature. Third, the use of two small motors ratherthan a single, larger motor simplifies vehicle weight distribution.Fourth, a dual motor drive system allows the drive system to beoptimized for a variety of different operating conditions, for exampleby using one motor for continuous light load operation with highefficiency and the other motor to supply high load or high speedsupplemental power. Such an approach allows improvements in performance,efficiency, and driving range to be achieved. For example, if a singlelarger motor drive system were to be utilized to supply the same fullload and full power (e.g., during highway hill climbing), its efficiencyat light load and low power (e.g., during city cruising, which mayrequire as little as 10% of full load and full power) would be muchlower than that of a smaller motor drive system sized for light load andlow power operations. Fifth, by coupling the motors to different ESSsystems, further improvements may be achieved in terms of weightdistribution and cooling efficiency as well as overall optimization ofthe ESS systems to reduce total weight, size, cost, stresses and aging.Sixth, the dual motor approach may be used to provide drive trainredundancy, thus improving vehicle reliability and performance.

FIG. 9 illustrates a first preferred embodiment of the invention thatutilizes a pair of ESS systems. As shown, motor 601 is connected to afirst ESS 901 via inverter 903 and power control module 905. Powercontrol module 905 is used to insure that the power delivered to motor601 or the regenerated power recovered from motor 601 has the desiredvoltage, current, waveform, etc. Similarly, second motor 603 isconnected to a second ESS 907 via a second inverter 909 and a secondpower control module 911. The power control modules may be comprised ofpassive power devices (e.g., transient filtering capacitors and/orinductors), active power devices (e.g., semiconductor and/orelectromechanical switching devices, circuit protection devices, etc.),sensing devices (e.g., voltage, current, and/or power flow sensors,etc.), logic control devices, communication devices, etc. In at leastone embodiment, power control modules 905/911 are under the control of acentral power control module 913. Preferably each inverter 903/909includes a DC to AC inverter.

As described above and shown in FIG. 9, each inverter 903/909 is coupledto its own ESS. Using dual ESS systems provides several benefits. First,the two ESS systems can be separately located within the vehicle, thusaiding in weight distribution. Second, each ESS system can have asmaller charge capacity than that which would be required by a singleESS system coupled to two motors. Third, each ESS system can be designedto meet the specific requirements of the motor to which it is coupled,thus potentially allowing differently sized ESS systems to be used,depending upon the requirements of the associated motors. Fourth, thecharging and discharging characteristics of the two ESS systems can bedesigned to be significantly different from one another. For example,the maximum charge and discharge rates of one of the ESS systems may bemuch higher than those of the other ESS. Preferably in at least oneembodiment, the minimum charge rate of the ESS 901 is 3C, where “C” isthe full capacity of the ESS divided by 1 hour in accordance withstandard conventions.

An important feature of drive system 900 is a bi-directional DC/DCconverter 915. DC/DC converter 915 provides a means for transferringenergy in either direction between the two drive systems. DC/DCconverter 915 is coupled to, and controlled by, an energy transfercontrol module 917. Energy transfer control module 917 monitors thecondition of each ESS system, for example monitoring the state of chargeof ESS 901 with sensor 919, and monitoring the state of charge of ESS907 with sensor 921. In at least one embodiment, energy transfer controlmodule 917 is configured to maintain one or both ESS systems within apreferred state of charge range, i.e., between a lower state of chargeand an upper state of charge. For example, energy transfer controlmodule 917 can be configured to maintain one or both ESS systems betweena lower limit and an upper limit, where the limits are defined in termsof a percentage of the maximum operating capacity of each respective ESSsystem. In at least one preferred embodiment, the limits for one or bothESS systems is 50% of the maximum operating capacity for the lower limitand 80% of the maximum operating capacity for the upper limit.

Preferably energy transfer control module 917 also monitors thetemperature of ESS 901 with a temperature sensor 923, and monitors thetemperature of ESS 907 with a temperature sensor 925. In at least oneembodiment, energy transfer control module 917 also monitors centralpower control module 913, thereby monitoring the requirements beingplaced on the two drive systems.

As outlined below, bi-directional DC/DC converter 915 providesoperational flexibility, and therefore a number of benefits, to variousimplementations of system 900.

-   -   i) Reserve Power—Bi-directional DC/DC converter 915 provides a        path and means for one drive system to draw upon the energy        resources of the other drive system when additional energy        resources are required. As a result, the ESS systems can be        designed with smaller charge capacities than would otherwise be        required.    -   For example, under normal operating conditions one of the motors        (e.g., motor 601) may only be required to supply a minor amount        of torque/power, therefore requiring that ESS 901 have only a        relatively minor capacity. However, under conditions when        additional torque/power assistance from motor 601 is required,        system 900 allows motor 601 to draw from ESS 907 via DC/DC        converter 915, power control module 905 and inverter 903.        Without converter 915, each ESS system would have to be designed        with sufficient energy capacity to handle the expected demands        placed on the system during all phases of operation.    -   ii) ESS Design Flexibility—Due to the inclusion of the        bi-directional DC/DC converter 915, the ESS systems can be        designed to optimize parameters other than just charge capacity.        For example, in at least one embodiment ESS system 901 utilizes        a supercapacitor module while ESS system 907 utilizes a        conventional battery pack, e.g., one comprised of batteries that        utilize lithium-ion or other battery chemistries. Bi-directional        DC/DC converter 915 allows system 900 to take advantage of the        benefits of each type of energy storage device without being        severely impacted by each technology's limitations.    -   iii) Charging Flexibility—During vehicle operation, preferably        regenerative braking is used to generate power that can be used        to charge either, or both, ESS systems 901 and 907. In system        900, bi-directional DC/DC converter 915 allows the electrical        power generated by either, or both, drive systems to be used to        charge either, or both, ESS systems. As a result, the state of        charge of both systems can be optimized relative to the        available power.    -   Although preferably both drive systems are used to generate        power, in at least one configuration only one of the drive        systems is used to generate electrical power via regenerative        braking as well as provide drive power. In such a configuration,        bi-directional DC/DC converter 915 allows the power generated by        the single drive system during the regenerative braking cycle to        be used to charge both ESS systems as required.    -   In addition, in a system such as that shown in FIG. 9, the two        ESS systems can utilize different charging profiles based on,        and optimized for, their individual designs. For example, one of        the ESS systems may be configured to accept a fast charging        profile. Since the two ESS systems are isolated, except for the        bi-directional DC/DC converter 915, the fast charging ESS system        is not adversely affected by the slowing down effect of the        other ESS system.    -   iv) Independent ESS/Drive System Design/Implementation—The        inclusion of the bi-directional DC/DC converter 915 provides        additional flexibility in the design and optimization of the        drive systems associated with each ESS system, for example        allowing drive motors with different nominal voltage levels to        be used.

FIG. 10 illustrates a second preferred embodiment of the invention. Asshown, system 1000 is the same as system 900 except for the eliminationof bi-directional DC/DC converter 915 and associated hardware.Eliminating the DC/DC converter effectively separates the electricalpower aspects of the two drive systems. As a result, ESS systems 901 and907 are designed to meet the expected needs of motors 601 and 603,respectively.

FIG. 11 illustrates a third preferred embodiment of the inventionutilizing a single ESS 1101 as in system 600 shown in FIG. 6. Thisillustration provides additional detail, specifically inverters 903/909,power control modules 905/911, and central power control module 913.Clearly in this embodiment the ESS system must have sufficient capacityto meet the expected needs of both motors 601 and 603.

FIG. 12 illustrates a fourth preferred embodiment of the invention. Asshown, system 1200 is the same as system 1100 except for the addition ofa DC/DC converter 1201 between ESS system 1101 and power control module911/inverter 909. DC/DC converter 1201 allows motor 601 to have a DC busnominal voltage range that is different from that of motor 603. It willbe appreciated that a DC/DC converter could also be interposed betweenESS 1101 and power control module 905/inverter 903, rather than betweenESS 1101 and power control module 911/inverter 909 as shown.

As described below in further detail, another aspect of the inventionthat is applicable to the dual drive system, regardless of ESSconfiguration, is a torque control system. This aspect is illustrated inFIG. 13 for a single ESS system such as that shown in FIG. 11. As shownand as previously described, motor 601 is connected to ESS 1101 via DCto AC inverter 903 and power control module 905. Similarly, motor 603 isconnected to ESS 1101 via DC to AC inverter 909 and power control module911. The power control modules 905/911 are used to insure that the powerdelivered to motors 601/603 or the regenerated power recovered frommotors 601/603 have the desired voltage, current, waveform, etc.

As previously noted, although FIG. 13 shows a single ESS, other ESSconfigurations such as those described above (e.g., dual ESSconfigurations) may also be used with the torque controller describedherein.

In accordance with the invention, system 1300 includes a torquecontroller 1301 that determines the power, i.e., voltage, current, andwaveform, that each of the power control modules 905/911 supplies totheir respective motors, and thus the torque and power that each motorapplies to axle 605. In order to calculate the appropriate power to besupplied to each motor, torque controller 1301 is coupled to, andreceives data from, a variety of sensors throughout the vehicle. Ingeneral, these sensors can be divided into four groups; those used tomonitor vehicle performance, those used to monitor the drive system,those used to monitor the condition and performance of the ESS(s) andthe power control electronics, and those used to monitor user input. Adescription of exemplary sensors for each group of sensors follows.

Vehicle Performance Sensors—The sensors within this group monitor theon-going performance of the vehicle by monitoring wheel spin, and thustire slippage, using one or more wheel spin sensors. In the illustratedembodiment, a wheel spin sensor is coupled to each wheel of each axle,i.e., sensors 1303-1306. The system may also include a vehicle stabilitycontrol system 1335 that detects vehicle spinning and then selectivelycontrols the vehicle's brake system in order to minimize such spinning.Vehicle stability control system 1335 may also control the torque ofmotor 601 and/or motor 603 during such an event via a stability torquerequest, thereby further enhancing vehicle control.

Drive System Sensors—The sensors within this group monitor theperformance of the two motors. Preferably coupled to motor 601 is atemperature sensor 1307 and a motor speed sensor 1309, and coupled tomotor 603 is a temperature sensor 1311 and a motor speed sensor 1313.

ESS and Power Control Electronics Sensors—The sensors within this groupmonitor the condition of the ESS and power control modules. Preferablycoupled to ESS 1101 is a temperature sensor 1315, a voltage sensor 1317and a current sensor 1319. Preferably coupled to power control module905 is a temperature sensor 1321. Preferably coupled to power controlmodule 911 is a temperature sensor 1323.

User Input Sensors—The sensors within this group monitor user input.Exemplary sensors in this group include a brake sensor 1325, anaccelerator sensor 1327, and a steering sensor 1329. These sensors canbe coupled to the corresponding pedals and/or steering wheel, coupled tothe corresponding linkage, or otherwise coupled to the vehicle drivesystems such that braking, accelerator and steering data is obtained.The system may also include a gear selection sensor 1331 if the vehicleincludes a multi-gear transmission, as opposed to a single speedtransmission. The system may also include a mode selection sensor 1333if the vehicle allows the user to select from multiple operating modes,e.g., high efficiency mode, high performance mode, etc.

Although the primary sensors used by torque controller 1301 are shown inFIG. 13 and described above, it will be appreciated that the inventioncan use other sensors to provide additional information that can be usedto determine the optimal torque split between the two motors. Forexample, by monitoring vehicle incline, the system can adapt for steephill climbing or descending conditions.

As previously noted, the present invention is not limited to vehiclesystems in which both motors are coupled to a single ESS. For example,FIG. 14 illustrates a torque control system similar to that shown inFIG. 13, with the exception that each motor/power control module iscoupled to a separate ESS. Specifically, motor 601 and power controlmodule 905 are coupled to ESS 1401 while motor 603 and power controlmodule 911 are coupled to ESS 1403. In this embodiment ESS 1401 includestemperature, voltage and current sensors 1405-1407, respectively, andESS 1403 includes temperature, voltage and current sensors 1409-1411,respectively. If desired, ESS 1401 can be coupled to ESS 1403, forexample using a bi-directional DC/DC converter (not shown) as describedin detail above.

FIG. 15 provides a more detailed schematic of torque controller 1301. Asshown, data from the brake sensor 1325, accelerator sensor 1327, gearselection sensor 1331 (if the vehicle has multiple gears), modeselection sensor 1333 (if the vehicle includes multiple modes) andvehicle stability control system 1335 (if the vehicle includes astability control system) are input into the vehicle torque commandgeneration unit 1501. The computed vehicle speed, referred to herein as“C_vspeed”, is also input into the vehicle torque command generationunit 1501. C_vspeed is computed by the traction command generation unit1509. The output of unit 1501 is a total torque requirement request,referred to herein as “C_torque”. C_torque is the torque required fromthe combined motors.

The maximum torque available from the two motors, referred to herein as“C_maxtorque1” and “C_maxtorque2”, are calculated by the first torquelimiting unit 1503 and the second torque limiting unit 1505,respectively. The inputs to the first torque limiting unit 1503 are thedata from first motor temperature sensor 1307, first motor speed sensor1309, and first power control module temperature sensor 1321. The inputsto the second torque limiting unit 1505 are the data from second motortemperature sensor 1311, second motor speed sensor 1313, and secondpower control module temperature sensor 1323. Assuming a single ESSconfiguration, for example as shown in FIG. 13, ESS data input to bothunits 1503 and 1505 are the ESS temperature data from sensor 1315 aswell as the ESS voltage and current data from sensors 1317 and 1319,respectively. If each motor is coupled to its own ESS as illustrated inFIGS. 9 and 10, then the ESS data input into unit 1503 is from the ESScoupled to motor 601 and the ESS data input into unit 1505 is from theESS coupled to motor 603.

The torque required from the combined motors calculated by unit 1501,and the maximum available torque for the first and second motors,calculated by units 1503 and 1505 respectively, are input into theoptimal torque split unit 1507 as is the computed vehicle speed. Unit1507 optimizes the torque split between the two motors without takinginto account wheel slip, thus splitting the desired combined torque,i.e., C_torque, into an optimal first motor torque request and anoptimal second motor torque request, the split based solely on achievingmaximum operating efficiency within the limits of the available torquefor each motor.

The system of the invention uses a simple continuously running algorithmto determine the optimal torque split, as illustrated in FIG. 16. Asshown, initially C_torque, C_vspeed, C_maxtorque1 and C_maxtorque2 areread (step 1601). Next, temporary values for the torque for the firstmotor 601 (C_temptorque1) and for the second motor 603 (C_temptorque2)are determined, as well as values for the motor flux for first motor 601(C_flux1) and for second motor 603 (C_flux2). (Step 1603). This step isperformed by interpolating data from a look-up table, described infurther detail below, that contains optimal torque (i.e., T1 and T2) andoptimal flux values (i.e., F1opt and F2opt) based on vehicle speed andtotal requested torque. The temporary torque values set in step 1603,based on the look-up table, are then compared to the maximum availabletorque values (step 1605) calculated by torque limiting units 1503 and1505. If the temporary torque values are less than the maximum availabletorque values, then the temporary torque values are output as C_torque1e (first motor) and C_torque2 e (second motor); if the temporary torquevalues are greater than the maximum available torque values, then themaximum available torque values are output as C_torque1 e and C_torque2e. (Steps 1607 and 1609). The flux command values for the first motor,i.e., C_flux1, and the second motor, i.e., C_flux2, are also output instep 1609.

FIG. 17 illustrates the preferred algorithm used to generate thethree-dimensional look-up table utilized by the optimal torque splitunit 1507. In step 1701, a first loop is initiated in which vehiclespeed, W, is stepped through from a minimum value, Wmin, to a maximumvalue, Wmax, in steps of Wstep. In step 1703, a second loop is initiatedin which total vehicle torque, T, is stepped through from a minimumvalue, Tmin, to a maximum value, T_(max), in steps of Tstep. In step1705, a third loop is initiated in which the torque of the first motor,T1, is stepped through from a minimum value, T1min, to a maximum valuein steps of T1step. The maximum value in step 1705 is the smaller ofT1max and T.

In the next series of steps, steps 1707-1709, the optimum flux value,F1opt, for the first motor 601 is determined for each value of T1.Initially, for a given value of T1 the first motor flux F1 is steppedthrough from a minimum value, F1min, to a maximum value, F1max, in stepsof F1step. Then for each value of T1 and F1, a value for first motorinput power, P1, is calculated. Next, F1opt is determined, based onachieving the minimum input power, P1min.

In the next series of steps, steps 1711-1714, the optimum flux value,F2opt, for the second motor 603 is determined for each value of T1.Initially for a given value of T1, the corresponding value for thetorque of the second motor, T2, is determined, where T2 is equal to Tminus T1. Then the second motor flux F2 is stepped through from aminimum value, F2 min, to a maximum value, F2max, in steps of F2step.Next, the value for the second motor input power, P2, is calculated foreach value of T2 and F2. Lastly, F2opt is determined, based on achievingthe minimum input power, P2 min.

In step 1715 a minimum total motor input power, Pmin, is calculated,where Pmin is equal to P1min plus P2 min. Next, the smallest Pmin isfound for the value of T1 for this particular iteration of the T1 loop.(Step 1717) Lastly, for the smallest Pmin and the current T and W,values for T1, T2, F1 opt and F2opt are output. (Step 1719)

The traction control command generation unit 1509 provides severalfunctions. As input, data from each wheel spin sensor, e.g., sensors1303-1306, is fed into unit 1509. Additionally, data from first motorspeed sensor 1309, second motor speed sensor 1313, and steering sensor1329 are input into the traction control command generation unit. Usingthis data, unit 1509 calculates vehicle speed, C_vspeed, which is inputinto the vehicle torque command generation unit 1501 as previouslynoted. Unit 1509 also uses the motor speed data to provide errorchecking.

A primary function of unit 1509 is to calculate wheel slip ratios, thewheel slip ratio being the difference between the wheel speed and thevehicle speed, divided by the greater of the wheel speed and the vehiclespeed. After calculating the wheel slip ratio as a function of vehiclespeed, a wheel slip ratio is calculated. The wheel slip ratio must takeinto account that wheels 609 and 611 of axle 605 may experiencedifferent degrees of slip, and thus exhibit different slip ratios. For alimited slip differential, and in most other cases as well, preferablythe higher of the two wheel slip ratios is taken as the wheel slip ratiofor that axle.

In order to determine if the wheel slip ratio is greater than desired,the wheel slip ratio must be compared to a target wheel slip ratiocontained within a lookup table. The lookup table provides target wheelslip ratios as a function of speed and steering angle. The lookup tablecan be based on well known target ratios or, as is preferred, based ontest data obtained for that particular vehicle and vehicleconfiguration. The difference between the computed wheel slip ratio andthe target wheel slip ratio yields the computed slip error, referred toherein as “C_sliperror”. To prevent control chatter, preferablyhysteresis is incorporated into the comparator used in this calculationby means of a dead band, i.e., neutral zone. In addition to controllingchatter, the hysteresis band also allows for a small amount ofadditional wheel slippage, which may compensate for vehicle weightdynamic distribution and improve acceleration and decelerationperformance.

The computed slip error, C_sliperror, along with the values for theoptimized torque split, C_torque1 e and C_torque2 e, and the totalrequested torque, C_torque, are input into the first stage of thetraction control unit 1511. Details of unit 1511 are shown in FIG. 18.As shown, the first stage independently minimizes the wheel slip ratioerror using a feedback control system, for example using a lead-lagcontroller, sliding-mode controller, PID controller or other linear ornon-linear controller type. Preferably a PID controller is used for thecompensator 1801 in the first stage feedback control system. In thesecond stage of unit 1511, motor speed fast disturbances areindependently minimized using high pass filters 1803/1804 andcompensators (preferably PID controllers) 1805/1806. Motor speed fastdisturbances can be caused, for example, by sudden large reductions ofload torque on the motor shaft during an excessive wheel slip event, orby sudden large additions of load torque on the motor shaft from a stuckwheel.

Note that in this embodiment, the first stage traction control furtherincludes a transient torque hybrid feedforward and feedback controlcircuit that during vehicle transient operations modifies the amount oftorque to the drive axle and modifies the otherwise efficiencyoptimizing torque request to one of the two motors, through afeedforward controller block K1 and a feedback controller block K2. Theamount of feedback torque modification is the result of the controllerK2 responding to the difference between the driver torque request afterthe slip error minimizing controller 1801 and the first motor torquecommand, C_torque-C_torque1 e. The feedforward controller block K1 isdesigned to behave like a low pass unity-gain filter, while the feedbackcontroller block K2 is designed to behave like a high pass filter withzero low frequency gain. The feedback torque component throughcontroller K2 is zero when the torque request is fully met, with a zeroeffective wheel slip ratio error and with the maximum torque limits notin effect. The hybrid feedforward and feedback control enhances thevehicle performance, vehicle response to driver request and drivabilitywithout compromising traction control. In addition to having differentpower and efficiency characteristics for improved total systemsteady-state performance, the two motor drive systems can be designed tohave different dynamic response characteristics for improved totalsystem dynamic performance.

After the second stage of traction control, torque limiters 1807/1808independently limit the torque commands issuing from the second stagebased on C_maxtorque1 and C_maxtorque2. The output of the torquelimiters 1807/1808 are torque commands C_torque1 and C_torque2. Thetorque commands from the limiters and the flux commands, C_flux1 andC_flux2, from the optimal torque split unit 1507 are input into controlmodules 905 and 911 as shown in FIG. 15. Power control modules 905 and911 can use any of a variety of motor control techniques, e.g., scalarcontrol, vector control, and direct torque control. Vector controlallows fast and decoupled control of torque and flux. In at least onepreferred embodiment of the invention, the control modules utilize apulse width modulator (PWM) control circuit.

In some instances the torque and flux motor control commands may besubject to further limitation, specifically due to component overheatingand/or ESS power limitations. Such command limits may be applied by anadditional limiter circuit within the torque controller 1301, or withinthe power control modules as illustrated in FIG. 15. In general, suchlimiters monitor the temperatures of the motors via sensors 1307/1311,the temperatures of the power electronics via sensors 1321/1323, and thetemperature, voltage and current of ESS 1101 via sensors 1315/1317/1319.If multiple ESS systems are used, as previously described, then thetemperature, voltage and current of each ESS system are taken as inputsto the limiters. In at least one embodiment using a single ESS system,if the ESS temperature is above a threshold temperature, then thecommands to the motors are proportionally reduced. If the temperature ofa particular power control module or a particular motor is above itspreset temperature threshold, then the control commands sent to thatparticular motor are reduced. Preferably in such an instance the controlcommands sent to the non-affected motor are sufficiently increased toinsure that the total requested torque, C_torque, is met. The limitersmay rely on a look-up table that provides preset command reductions as afunction of the amount that a monitored temperature is above itsrespective preset temperature threshold.

In accordance with at least one preferred embodiment, the torque andtraction controller 1301 uses multiple processing frequencies, thespecific frequency depending upon the function of the unit in question.For example, a dual frequency approach can be used in which a relativelylow frequency is applied in order to optimize the performance of the twomotors based on general operating conditions, while a second, higherfrequency is applied in order to quickly respond to rapidly developingtransient conditions, e.g., wheel slippage. In this preferred approach,low frequency cycling is applied to the torque command generation unit1501, the torque limiting units 1503/1505, the optimal torque split unit1507 and the various temperature, voltage, current, and speed sensors.Preferably the low frequency is selected to be within the range of 100Hz to 2 kHz, more preferably in the range of 500 Hz to 1.5 kHz, and evenmore preferably set at approximately 1 kHz. High frequency cycling isapplied to the traction control unit 1511, control modules 905/911 andthe wheel slip sensors, and is preferably at a frequency of about 10 to30 times that of the low frequency, and more preferably at a frequencyof approximately 20 kHz. As the traction control command generation unit1509 monitors wheel slippage and generates the slip errors for eachaxle, preferably it operates at the high cycle frequency although in atleast one embodiment, it operates at an intermediate rate, e.g., 5-10kHz.

As previously noted, the present control system can be used with an EVthat utilizes a single ESS for both motors, or one which utilizes an ESSper motor. The system and methodology is basically the same aspreviously described in detail, except that the temperature, current andvoltage of each ESS must be monitored and taken into account. Thus, forexample, the control system shown in FIG. 15 would be modified as shownin FIG. 19. Specifically, the temperature, current and voltage of thefirst ESS 1401 would be sensed with sensors 1405-1407 and input into thefirst torque limiting unit 1503 and the first control module 905; andthe temperature, current and voltage of the second ESS 1403 would besensed with sensors 1409-1411 and input into the second torque limitingunit 1505 and the second control module 911.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

What is claimed is:
 1. A method of operating an electric vehicle, theelectric vehicle having a first electric motor coupled to a vehicledrive axle and a second electric motor coupled to said vehicle driveaxle, the method comprising the steps of: a) monitoring a wheel speedsensor corresponding to said vehicle drive axle, a first motor speedsensor and a second motor speed sensor, and computing a vehicle speedcorresponding to said electric vehicle based on output from said wheelspeed sensor, said first motor speed sensor and said second motor speedsensor, wherein said vehicle speed computing step is performed by atraction control command generation unit; b) monitoring a brake sensorand an accelerator sensor, and computing a total torque requirementrequest based on output from said brake sensor and said acceleratorsensor and said vehicle speed, wherein said total torque requirementrequest computing step is performed by a vehicle torque commandgeneration unit; c) splitting said total torque requirement request intoan optimal first motor torque request and an optimal second motor torquerequest, wherein said total torque requirement request splitting step isperformed by an optimal torque splitting unit; d) inputting a wheeltarget slip ratio and computing a slip error corresponding to saidvehicle drive axle based on output from said wheel speed sensor and saidvehicle speed and said wheel target slip ratio, wherein said first sliperror computing step is performed by said traction control commandgeneration unit; e) minimizing said slip error using a feedback controlsystem implemented by a traction control unit, said traction controlunit further performing the steps of transforming said optimal firstmotor torque request into a first motor torque command and transformingsaid optimal second motor torque request into a second motor torquecommand based on said step of minimizing said slip error; f) controllingsaid first electric motor based on said first motor torque command; g)controlling said second electric motor based on said second motor torquecommand; and h) repeating steps a)-g) throughout operation of saidelectric vehicle.
 2. The method of claim 1, further comprising the stepof monitoring a steering sensor, wherein said step of computing saidvehicle speed is based on output from said wheel speed sensor, saidfirst motor speed sensor, said second motor speed sensor, and saidsteering sensor.
 3. The method of claim 1, wherein said total torquerequirement request splitting step further comprises the step ofinterpolating data from a look-up table, said look-up table containingsaid optimal first motor torque request and said optimal second motortorque request as a function of said vehicle speed and said total torquerequirement request.
 4. The method of claim 1, step c) furthercomprising the steps of: monitoring a first motor temperature sensor;computing a first motor maximum available torque based on output fromsaid first motor speed sensor and said first motor temperature sensor;limiting said optimal first motor torque request by said first motormaximum available torque; monitoring a second motor temperature sensor;computing a second motor maximum available torque based on output fromsaid second motor speed sensor and said second motor temperature sensor;and limiting said optimal second motor torque request by said secondmotor maximum available torque.
 5. The method of claim 4, furthercomprising the steps of: monitoring a first power control moduletemperature sensor, said first power control module temperature sensorin thermal communication with a first power control module, said firstpower control module electrically connected to said first motor, whereinsaid first motor maximum available torque computing step is furtherbased on output from said first power control module temperature sensor;and monitoring a second power control module temperature sensor, saidsecond power control module temperature sensor in thermal communicationwith a second power control module, said second power control moduleelectrically connected to said second motor, wherein said second motormaximum available torque computing step is further based on output fromsaid second power control module temperature sensor.
 6. The method ofclaim 4, further comprising the steps of: monitoring a first energystorage system (ESS) temperature sensor, said first ESS temperaturesensor in thermal communication with a first ESS, said first ESSelectrically connected to a first power control module, said first powercontrol module electrically connected to said first motor; monitoring afirst ESS voltage sensor; monitoring a first ESS current sensor, whereinsaid first motor maximum available torque computing step is furtherbased on output from said first ESS temperature sensor, said first ESSvoltage sensor and said first ESS current sensor; monitoring a secondESS temperature sensor, said second ESS temperature sensor in thermalcommunication with a second ESS, said second ESS electrically connectedto a second power control module, said second power control moduleelectrically connected to said second motor; monitoring a second ESSvoltage sensor; and monitoring a second ESS current sensor, wherein saidsecond motor maximum available torque computing step is further based onoutput from said second ESS temperature sensor, said second ESS voltagesensor and said second ESS current sensor.
 7. The method of claim 4,further comprising the steps of: monitoring an energy storage system(ESS) temperature sensor, said ESS temperature sensor in thermalcommunication with an ESS, said ESS electrically connected to a firstpower control module and to a second power control module, said firstpower control module electrically connected to said first motor, saidsecond power control module electrically connected to said second motor;monitoring an ESS voltage sensor; and monitoring an ESS current sensor,wherein said first motor maximum available torque computing step andsaid second motor maximum available torque computing step are furtherbased on output from said ESS temperature sensor, said ESS voltagesensor and said ESS current sensor.
 8. The method of claim 1, furthercomprising the steps of minimizing first motor speed fast disturbancesand second motor speed fast disturbances using a second feedback controlsystem implemented by said traction control unit, wherein said steps oftransforming said optimal first motor torque request into said firstmotor torque command and transforming said optimal second motor torquerequest into said second motor torque command are based on said step ofminimizing said slip error and based on said step of minimizing saidfirst motor speed fast disturbances and said second motor speed fastdisturbances.
 9. The method of claim 1, step e) further comprising thesteps of: monitoring a first motor temperature sensor; computing a firstmotor maximum available torque based on output from said first motorspeed sensor and said first motor temperature sensor; limiting saidfirst motor torque command by said first motor maximum available torque;monitoring a second motor temperature sensor; computing a second motormaximum available torque based on output from said second motor speedsensor and said second motor temperature sensor; and limiting saidsecond motor torque command by said second motor maximum availabletorque.
 10. The method of claim 9, further comprising the steps of:monitoring a first power control module temperature sensor, said firstpower control module temperature sensor in thermal communication with afirst power control module, said primary power control moduleelectrically connected to said first motor, wherein said first motormaximum available torque computing step is further based on output fromsaid first power control module temperature sensor; and monitoring asecond power control module temperature sensor, said second powercontrol module temperature sensor in thermal communication with a secondpower control module, said second power control module electricallyconnected to said second motor, wherein said second motor maximumavailable torque computing step is further based on output from saidsecond power control module temperature sensor.
 11. The method of claim9, further comprising the steps of: monitoring a first energy storagesystem (ESS) temperature sensor, said first ESS temperature sensor inthermal communication with a first ESS, said first ESS electricallyconnected to a first power control module, said first power controlmodule electrically connected to said first motor; monitoring a firstESS voltage sensor; monitoring a first ESS current sensor, wherein saidfirst motor maximum available torque computing step is further based onoutput from said first ESS temperature sensor, said first ESS voltagesensor and said first ESS current sensor; monitoring a second ESStemperature sensor, said second ESS temperature sensor in thermalcommunication with a second ESS, said second ESS electrically connectedto a second power control module, said second power control moduleelectrically connected to said second motor; monitoring a second ESSvoltage sensor; and monitoring a second ESS current sensor, wherein saidsecond motor maximum available torque computing step is further based onoutput from said second ESS temperature sensor, said second ESS voltagesensor and said second ESS current sensor.
 12. The method of claim 9,further comprising the steps of: monitoring an energy storage system(ESS) temperature sensor, said ESS temperature sensor in thermalcommunication with an ESS, said ESS electrically connected to a firstpower control module and to a second power control module, said firstpower control module electrically connected to said first motor, saidsecond power control module electrically connected to said second motor;monitoring an ESS voltage sensor; and monitoring an ESS current sensor,wherein said first motor maximum available torque computing step andsaid second motor maximum available torque computing step are furtherbased on output from said ESS temperature sensor, said ESS voltagesensor and said ESS current sensor.
 13. The method of claim 1, whereinsteps a) through d) are repeated at a first frequency and steps e)through g) are repeated at a second frequency, wherein said secondfrequency is between 10 and 30 times said first frequency.
 14. Themethod of claim 1, wherein steps b) and c) are repeated at a firstfrequency, steps e) through g) are repeated at a second frequencybetween 10 and 30 times said first frequency, and wherein steps a) andd) are repeated at a third frequency, said third frequency between saidfirst and second frequencies.
 15. The method of claim 1, furthercomprising the step of computing an optimal first motor flux command andan optimal second motor flux command, wherein said step of computingsaid optimal first motor flux command and said optimal second motor fluxcommand further comprises the step of interpolating data from a look-uptable, said look-up table containing said optimal first motor fluxcommand and said optimal second motor flux command as a function of saidvehicle speed and said total torque requirement request.